The first splitting variable is a categorical variable (here named cat,
which contains value 0 through 9 and is a factor), but the output does not
specify which values went into which part of the tree:

1) cat == {}; criterion = 1, statistic = 32.792

Can anyone help me to get the detail on this splitting variable to appear
in the output?

I regret that I cannot send a reproducible example because the data is
proprietary. I will try to work up an example with a public data set that
has the same problem.